Air-distribution system



July 30, 1963 c. E. HARRISON AIR-DISTRIBUTION SYSTEM 2 Sheets-Sheet 1 Filed July 14, 1960 :m Zgfgg July 30, 1963 c. E. HARRISON AIR-DISTRIBUTION SYSTEM Filed July 14, 1960 Fig. 4

2 Sheets-Sheet 2 A v A Patented July 30, 1963 ice 3,099,200 AIR-DISTRIBUTION SYSTEM Charles E. Harrison, Broadwater Farm, Charlestown, Pa.

Filed July 14, 1960, Ser. No. 42,918 14 Qlahns. (CI. 98-40) 'Ilhis invention relates to air-distribution systems in which air is supplied through a ceiling or other wall to a building space from a pressurized plenum chamber.

In accordance with the present invention, the quantity of air supplied from the plenum chamber to each of various zones of the building space served by the plenum and the extent of penetration of such air into the zones is locally controlled to meet the different lair requirements of the various zones in these respects. More particularly, the flow of air from the plenum chamber to the building space is through a plurality of flow-control devices which are dispersed in the ceiling or other wall area. in a pattern suited to meet the air requirements of the different zones and which have provision for obtaining different output velocities of the effluent air at a substantially constant air pressure maintained throughout the plenum.

More specifically, the devices for controlling passage of air from the plenum chamber to the served zones have flow passages of different flow resistances to obtain dif- -feren-t velocities of their effiuent air streams. In preferred forms of the flow-control device, exists for the efliuent are spaced along a flow-resistance passage whose crossscctional area may be adjusted to change not only the average velocity of the efi iuent air but also the velocity gradient across the flow-resistance passage. In preferred forms of the control device, it is provided with valve means adjustable to vary the area of the air inlet to the flow passage independently of adjustment of the flow resistance of that passage.

The invention further resides in air-distribution systems and flow-control devices having features of construction, combination and arrangement hereinafter described and claimed.

A more complete understanding of the invention may be had firom the following description in which reference is made to the accompanying drawings, in which:

FIG. 1 is a perspective view illustrative of an air-distribution system embodying the invention;

FIG. 2 is a side elevational view, in section, of a flowcontrol device suited for inclusion in the ceiling t FIG. 1;

FIG. 2A is a modification of FIG. 2 with provision for control of direction of the effluent air;

FIG. 3 is a top plan view, on reduced scale and in part broken away, of the flow-control device of FIG. 2;

FIG. 4 is a side elevational View, in section, of another type of flow-control device suited for use in the installation of FIG. 1; and

FIGS. 5 and 6 show the flow-control devices with auxhe-at-exchrange provisions.

Referring to FIG. 1, the building space 10 defined by ceiling 11, floor 12, side walls-13A, 13B and end walls 14D, 14E is composed of various zones having different requirements as to air movement and/ or the quantity of conditioned air supplied to them for comfort of personnel and/or protection of equipment or materials and/ or suited fora particular manufacturing process. In this particular installation, air for such purposes is supplied from a plenum chamber 16 which is above and substantially ooextensive with the ceiling 11 through a multiplicity of flowcontrol devices 17 distributed in the ceiling area. (The reference characters 17 common to all of the flow devices shown have different suflixes for different zones.) The lower wall of plenum 16 may be and usually is formed by the ceiling 11; its upper wall may be and usually is the roof or upper floor 18 of the building; and the side and end walls of the plenum may be and usually are formed by the upper portions of the side and end walls of building space 10. Except for the usual beams, girders or other supporting structure (omitted firom FIG. 1 for purposes of clear illustration), the plenum chamber is essentially a large storage space for air substantially coextensive with the zones served by it and is maintained at substantially constant positive pressure by continuous supply of air firom duct 19 connected to air-conditioning equipment, not shown, which heats, cools and cleans the air and which may also humidify the air.

For substantial uniformity of the plenum pressure throughout the extensive ceiling area, adequate sealing precautions should be taken to minimize leakage from the plenum chamber and the total open discharge area of the flow-control [devices 17 should not exceed more than about 2% of the ceiling area.

As apparent from FIG. 1, the flow'control devices 17 are distributed in a pattern, which as will appear, is correlated to the different air-load requirements of various zones of the building space 10 served from the plenum. Along the ceiling area adjacent wall 13A, which is an exposed wall largely composed of window areas, there is a row or group of flow-control devices 17A spaced to provide large-volume air streams moving downwardly from the ceiling openings at relatively high velocities for deep penetration into the window or other cold wall zones and merging well above the floor level to minimize temperature differentials. By way of contrast, the row or group of flowcontro1 devices 17B for supplying air to the zone B extending along the blind or inner wall 13B may be fewer in number than group 17A and spaced farther firom the wall and from one another. The flow devices 17B deliver a total quantity of air to their zone which is substantially less than supplied along the opposite exposed wall zone A. Also by design or adjustment, the velocities of the air streams from devices 17B may be lower than for devices 17A. If any subdivision of zone B requires a greater or lesser amount of conditioned air and/or different depth of penetration because of equipment or materials therein disposed, one or more flow devices 17B supplying that subdivision may be adjusted, or preset, as later described to meet the particular air requirements of such subdivision.

Except near the service door 20 in remote end wall 14D, the corridor zone C extending lengthwise of the building space has a much lower air-load requirement than for example zone A along the exposed side wall 13A. Such lower air requirements may he satisfied by the row or group of flow devices 17C spaced more widely than control devices 17A along wall 13A. Also, the individual flow devices along the corridor zone may be adjusted or preset to provide lower velocity of their effluent air 'for lower penetration into the corridor space.

At the tar end of corridor C, the air-load requirement is high because of proximity to the service door 2i) which may of necessity be opened tfOl appreciable periods of time. The air requirement of this zone may be supplied from the plenum through a group of flow devices 178 whose number is greater than for equivalent ceiling area of the zones above discussed. Also by adjustment or presetting, the depth of penetration of the air streams from devices 178 into the building space may be even greater than for zone A along exposed wall 13A. At the near end of corridor C which is closed by blind wall 14E, the airload requirement is low and may be satisfied by a relatively small number of flow-control devices 17E supplying a correspondingly less quantity of air. If required, the individual devices 17E may be adjusted or preset to supply air at different average velocities if air turbulence is required at different levels to satisfy the conditioning requirements of business or manufacturing machines disposed in subdivisions of this zone of the building space 10.

In the reception room zone R, entered from the exterior of the building through the main door 21, the heavy air requirements near the window is met by that fraction of the total number of flow-control devices 17A which is common to the two zones. The still heavier air requirement near the door 21 issupplied from the group of control devices 17M, but since this door is usually used less frequently or left open for shorter periods than service door 29, the number in this group may be less than in group 175. The air requirements of the rest of the reception zone may be satisfied by the group of flow devices 17R respectively suitably adjusted or preset to provide the quantity and turbulence of air suited to the particular uses of corresponding subdivisions of the zone. For example, at a receptionists desk near the center of the zone, lesser volume of air but greater penetration than along the walking area from door 21 to corridor C may be desired.

The nearest row of flow-control devices 17R above the visitors seating area may provide a substantial quantity of conditioned ai at substantially uniform velocity afiording l-ow turbulence atseated breathing level.

In the ofiices N and P, differing in size and occupancy, the diiierent total air requirements are respectively satisfied from the plenum by a correspondingly different number and distribution of the flow-control devices. In the smaller otfice N, the total :air requirements may be satisfied by only a few control devices l'iN in addition to the flow-control devices 17A for the window area, whereas for the larger oflice P, a larger number of additional air-flow devices 17? is necessary.

For the shop zone W, a row or group of air-flow devices 17W along the rear and side blind walls above benches and machines provide the air required in addition to that supplied by the flow devices 17A in the ceiling along thewindow area. The quantity of air supplied by the individual devices 17W and the velocity of such air may be adjusted or preset to satisfy the particular air requirements t difierent portions of the shop.

Provisions for return of air to the conditioning equipment' and addition of make-up air may be in accordance with known practice. In the installation shown in FIG. 1, the duct 19A may be used for such purpose. It will be noted that such return duct is remote from any of the flow devices in avoidance out short-circuiting air from the supply plenum.

From the foregoing it will be understood that many and varied air requirements of the building space It may be satisfied both as to quantity of air and air velocity from a plenum chamber co-extensive with the zones served by it and maintained at substantially constant presure, rather than by a network of ducts expensive to install and diihcult to adjust. It will also be appreciated by those skilled in the art that such widely varied air requirements are not and cannot be satisfied by the previously extensively used perfiorated ceiling tile arrangements providing a blanket of air descending uniformly throughout the ceiling area.

As exemplary of flow-control devices suited for use in Al the air-distribution system of FIG. 1 and for other installations requiring local control of the quantity and velocity of air supplied from a plenum chamber to different zones of a building space, reference is first made to the type of flow-control device X17 shown in FIGS. 2 and 3. As indicative of the order of its size, for most installations such device would be of area equivalent to a circle having a radius in the range of 4 to 12 inches.

The lower plate 25' of the device extending across a ceiling opening is of dished or conical shape and is joined at its periphery to an upper plate 26, which is also of dished or conical shape and of somewhat greater radius of curvature or of greater apex angle. Both plates are usually of metal. The lower plate has a multiplicity of perforations or holes 27 distributed throughout a sub-stantial part of its area. The upper plate 26 has a central open area 28 exposed to the air in the plenum space 16. As apparent from 'FIG. 2, the space between the two plates is a flow passage 29 which converges outwardly from inlet opening 28 in the upper plate 26 toward the outer joined edges of the plates 25 and 26. Because of such convergence of the flow path, its resistance progressively increases toward discharge holes 27 which are more and more remote from the air inlet 23.

Because of progressively increasing pressure drop radially outwardly from the air inlet 28, the air stream discharged downwardly from the fiow device X17 has maximum velocity at its center and progressively lower velocity toward its perimeter generally as shown by the solidline velocity vectors V V This shape of air discharge is desirable because although the average velocity may be high for delivery of a substantial volume of air well down into the building space, the stream velocity near the adjacent ceiling area is low, so avoiding smudging by dustladen air which otherwise would be induced to flow there.

The different air penetration requirements of various flow devices 17 of FIG. 1 may be satisfied by making them of the construction shown in FIG. 2 as thus far described with different fixed convergences of the upper and lower plates. As now described, any given size of flow device maybe adapted to meet a wide range of penetration requirements by making plate 26 in the form of a diaphragm which can be adjustably flexed to vary the flow resistance of passage 29 to the outlet openings 27 spaced along it. Specifically and by way of example, the plate 26 may, as shown in HS. 2, be of thin metal corrugated to ailord extensibility and at its center may be provided with a threaded stud 30 extending through an adjusting knob or nut 31 accessible from the building space 1%) below device X17. If required, a biasing spring 32s may be interposed between the two plates to supplement the natural restoring force of diaphragm plate 26.

Still assuming a fixed inlet area to the flow passage, the effect of tightening the adjusting knob 31 or otherwise reducing the angle of convergence between the plates 25 and 26 is to force a greater percentage of all of the admitted air out of discharge holes 27 which are nearer the center of the device, generally as indicated by the dottedline vectors f -V of FIG. 2. 'In consequence, for the same plenum pressure, the peak velocity of the efliuent air is increased for greater penetration of the discharged air into the building space; such greater penetration is without corresponding increase in the total quantity of air supplied. The foregoing is upon the assumption that the total outlet area of the perforations in plate 25 is generally not less than, and is usually of the same order of magnitude as, the elfective area of the inlet. opening in plate 26 and that the outlet area increases in direction of convergence of the flow passage 29. These conditions are satisfied by the construction shown in FIGS. 2 and 3, it being noted that the number of perforations 27 in the circumferences of successive circles (-FIG. 3) increases with increase in radius of the circles. In any event, the total area of the perforations near the center of plate 25 should be significantly less than the total area of the central inlet opening 28 in avoidance of channeling and loss of control by variation of the resistance of flow passage 29.

From manufacturing and installation standpoints, it is also desirable that any one size of flow-control device 17 be suited to supply air over a considerable quantity range by adjustment either at the factory or at time of installation. To that end, the inlet opening '28 of device X17 is designed to pass the maximum quantity of air required of a single flow device at the usual minimum plenum pressure, usually about 0.01 inch of water, and there is provided a valve plate 32 or equivalent for adjusting the effective area of such opening to supply the normal minimum of conditioned air at a usual maximum plenum pressure about 0.15 inch of water. The uniform pressure maintained in the plenum will differ for different installations and at different times will differ in the same installation if the total air requirements of the zones served by the plenum are changed.

in the form shown in FIG. 2, the adjustable valve plate 32 is retained in seating engagement with plate 26 by headed pins 33 which extend through angular slots 34 in plate 32 and plate 32 is provided with a stud 35 extending into the bore of hollow stud 30 and having a screwdriver slot for adjustment from the building space It The knob 31, or equivalent, and the valve stud 35, or equivalent, may be calibrated for a given plenum pressure in terms of air-quantity and air-penetration respectively.

By inserting a multiplicity of such flow-control devices X17 in predetermined pattern in a ceiling area, the various zones of the builidng space served from the plenum and having specific requirements differing as to quantity of air and air-penetration may have such requirements satisfied from a common plenum chamber for the zones by controlling the flow resistance and/or the inlet openings of the various flow-control devices of such construction.

To avoid down drafts in the vicinity of outside 'window areas and other similar building areas when large volumes of conditioned air at relatively high velocity should wash the wall areas, the device of FIG. 2 may be provided with means for imparting a lateral component of movement to the effluent air. Specifically as shown in FIG. 2A, the flow-control device may be recessed into the plenum space by mounting it upon an imperforate ring 40 or equivalent surrounding the ceiling opening. Attached to this mounting below the lower discharge plate 25 is a plate 41 having a series of discharge slots and a series of hinged vanes 42. By tilting the vanes, the air streams from the perforated plate 25 may be deflected toward an adjacent side Wall. The vanes may be divided as shown into two groups with independent control knobs 43 accessible adjacent the knob 31 which controls the inlet valve 32.

It is to be observed that in FIG. 2A the area of the opening into the flow passage 29 is varied by an adjustable fiow-controlling element 32A which, as shown, may be moved upwardly and downwardly by the threaded element 25A. When in its dotted-line position, the element 32A closes the opening into the flow passage 29, while in its fully elevated position, substantially entirely the whole cross-sectional area of the cylindrical opening is available for passage of air into the passage 29.

This modification of the flow-control device of FIG. 2 is particularly suited in the installation of FIG. 1 for use as the flow devices 17A along the exterior window wall 13A.

With lesser independence between control of air-quantity and air-penetration, the flow-control devices Y17 of FIG. 4 may be used. In this construction, like that of FIG. 2, the lower plate 25A is of bowl or cone-shape with a multiplicity of perforations 27 providing for discharge of air into the building space at different velocities because spaced along a flow-resistance passage. In FIG. 4, however, the flow passage 29A diverges outwardly from the center of the device. The apex angle or radius of curvature of the lower face of the upper structure or plate 26A is less than that of lower plate 25A to provide 6 convergence of the path of flow inwardly from the inlet opening 28A between the spaced peripheries of the plates.

The flow resistance of the device Y17 may be varied by moving the structure 26A vertically with respect to discharge plate Specifically, the plate or structure 26A may be provided with a threaded central hub A which receives an adjusting bolt 31A. The latter extends through plate 25A and is prevented from moving vertically with respect thereto by pinned washer 37 or equivalent and the bolthead. Rotation of plate 26A during such vertical adjustment is prevented as by pins 36 extending from plate 25A through guide holes in structure 26A.

Lowering of the plate structure 26A to increase the pressure drops along the flow passage 29A also reduces the area of the peripheral inlet opening 28A. Thus, the average air velocity of the stream and the total quantity of air in it are concurrently reduced. In other Words, the controls of air-quantity and air-penetration are interlocked with consequent limitation which must be considered in selection of where the flow-control devices Y27 are to be used in a given distribution system. Also as indicated by the velocity vectors V V of FIG. 4, with this type of flow-control device, the air velocity is at its maximum about its perimeter with consequent possibility of ceiling smudging which is objectionable in oflice, restaurant andsimilar zones, though of minor concern in a shop or storage room.

Both types of flow-control devices above described are satisfactory for use or incorporation with a heatexchanger as a unit for supplemental heating or cooling of air supplied to a particular zone or subdivision thereof. For example, the device X17, as shown in FIG. 5, may have mounted upon it a finned tubing array 45 traversed by a heating or cooling medium for heating or cooling the air immediately before or during its passage to air inlet 28 of the fiow-control device. As indicated in FIG. 6, the flow-control device Y17 may support adjacent its circumferential air inlet 28 a finned helix or grid 46 whose piping is traversed by a heating or cooling medium. Also with their upper and lower plates made of translucent material such as some of the high-strength plastics, both of these types of flow devices may serve as the bottom of a luminaire such as used in recessed lighting systems. This simplifies the ceiling construction, enhances its appearance, and localizes the air requirements related to lighting which are not insubstantial with present high levels of illumination.

The term ceiling as herein used shall be interpreted to mean the bottom of the plenum Whether or not there is disposed below it a supplementary or false ceiling of relatively open construction such as egg-crate construction commonly used in the so-called luminous ceiling. The invention is also applicable to floor or side walls with the plenum on that side of the Wall which is opposite to the building space supplied from the plenum. It is also applicable where gases other than air are sup plied: the term air is used generically in the following claims.

Now that the principles of the invention have been set forth, it will be understood that many modifications may be made within the scope of the appended claims. For example, any suitable means may be utilized to vary the cross-sectional area of the flow passage of each flowcontrol device with and without the further adjustment of the area of the air inlet from the plenum chamber to each of the flow passages. While variation of one or both boundary walls of the flow passage or other wall structure thereof will be effective to vary the cross-sectional area and thus to adjust the average air velocity as well as the velocity gradient across that passage, variations in the cross-sectional area of the flow passages may be made in other ways, as for example, by providing spiral flow paths with adjustable walls which may then be movable relative to each other to achieve the same accuses desired changes in average velocity and velocity gradient.

What is claimed is:

1. An air distribution system for supplying different predetermined quantities of air to different zones of a building space and with predetermined difierent depths of penetrations into different zones comprising a Wall common to said different zones, a plenum chamber substantially coextensive with said wall and continuously supplied with air for maintenance of positive plenum pressure, and a plurality of air-control devices inserted in said Wall in a pattern dependent upon the different airload requirements of different zones and jointly having a discharge area not exceeding about 2% of the area of said wall for substantial uniformity of the plenum pressure at all of said air-control devices, said control devices each comprising a perforated plate structure bridging an opening in the wall and structure, such as a plate, in the plenum chamber and having a surface variably spaced along said perforated lower plate to *form a how passage converging in the direction of flow of air from the plenum through the perforations in said plate and determining the flow resistance of the air-flow paths to said perforations, at least some of said control devices having means for adjusting the inter-plate spacing.

2. An air-distribution system as in claim 1 in which in at least some of said control devices the two structures are joined at their peripheries with increasing spacing toward their centers and in which the plate structure in the plenum has a central inlet for admission from the plenum of air which flows through the apertured plate in paths of progressively increasing resistance.

3. An air-distribution system as in claim 1 in winch in at least some of said control devices the plate structures are spaced at their outer edges to provide an inlet for air from the plenum chamber and in which the spacing decreases toward the center of the plate structure progressively to increase the pressure drop in the air path to perforations nearer and nearer to the center of the other plate structure.

4. An air-distribution system for a building space comprising a wall for said space and coextensive therewith, a plenum chamber substantially coextensive with said wall and continuously supplied with air for maintenance of positive plenum pressure, and a plurality of air-flow devices distributed in the wall in accordance with the different air-requirements of different zones and having a total air discharge area not exceeding about 2% of the area of said Wall, each of said devices comprising a first plate bridging a wall opening with a multiplicity of airdischarge perforations distributed over its area and a second plate in the plenum chamber, said plates converging in the direction of flow of air from the plenum toward perforations in the second plate which are more and more remote from the region of admission of the plenum air to the flow passage formed by said plates to provide air-flow paths of progressively higher flowresistance.

5. An air-distribution system for a building space comprising a wall for said space and coextensive therewith, a plenum chamber coextensive with said wall and continuously supplied with air for maintenance of positive plenum pressure and a plurality of air-flow devices distributed in the wall in accordance with the different airrequirements of different Zones and having a total airdischarge area not exceeding about 2% of the area of said wall, each of said devices comprising two spaced plates joined at their peripheral areas and closing an opening in the wall, one of said plates having an opening substantially spaced from its perimeter for admission of air from the plenum to the space between the plates, the other of said plates having a multiplicity of perforations distributed over its area for exit of air from the interplate space to the building space, the spacing between said plates decreasing from the air admission opening toward the peripheral joinder of the plates for discharge of air from the perforations in the second-named plate at velocities decreasing toward the boundary of said wall opening.

6. An air-distribution system as in claim'S, in which the first-named plate of'individual air-flow devices is a diaphragm adjustably extensible for adjustment of the convergence of the spaced plates, and structure for adjusting said diaphragm for control of the velocity gradient transversely of the effluent air discharged from the perforated plate.

7. An airadistribution system as in claim 6 in which the air-admission opening in the diaphragm is provided with adjustable structure for varying the quantity of air discharged from the perforated plate independently of the adjustment varying the interplate spacing for control of velocity gradient of the eflluent air.

8. An air-distribution system for a building space comprising a wall for said space and substantially coextensive therewith, a plenum chamber above and coextensive with said wall and continuously supplied with air to maintain positive plenum pressure and a plurality of airflow devices distributed in the wall and having a total air-discharge area not exceeding about 2% of the area of said wall, each of said devices comprising two structures having opposed faces spaced to provide an annular flow passage progressively decreasing in cross-sectional area from the spaced peripheries of said structures, one of said structures being in the plenum chamber and the other of said 5 ructures bein" a plate bridging an opening in the wall and having a multiplicity of perforations spaced radially and angularly in said flow passage.

9. An air-distribution system as in claim 8 in which means are provided to effect relative linear adjustment of said structures concurrently to vary the resistance of said flow passage and the air-admission area at the spaced eripheries of said structures.

10'. An air-control device for bridging an opening in a wall comprising a plate structure having a multiplicity of perforations distributed over its area, a second structure spaced from and cooperating with said perforated plate structure to define a flow passage having an opening for admission of air and converging in the direction of flow of air from said air-admission opening to perforations of said plate, said second structure being a diaphragm joined at its periphery to the perforated plate structure and having a substantially centrally disposed air-adniission opening, and means adjustably to hex the diaphragm to control the convergence of said air passage.

11. An air-control device as in claim 10 additionally having structure adjustable to vary the effective area of the central opening in the diaphragm to control the quantity of air discharged from the perforated plate independently of the adjustment of the diaphragm in control of the velocity gradient of the discharged air.

12.. An air-control device for bridging an opening in a wall comprising a plate structure having a multiplicity of perforations distributed over its area, a second structure cooperating with said perforated plate structure to define a how passage, the two structures being spaced at their peripheries to provide an inlet opening for admission of air to the flow passage which converges in direction away from said peripheries, and means to effect linear adjustment of the second structure for concurrent adjustment of the area of the inlet opening and of the resistance of said flow passage.

13. An air-distribution system comprising a plenum chamber having a wall with a plurality of spaced openings disposed in a pattern dependent upon the different airload requirements of different building space zones, and air-control devices for respectively bridging said openings, said air-control devices each comprising a plate structure having a multiplicity of perforations distributed over its area, a diaphragm joined at its periphery to said perforated plate structure and having a substantially centrally disposed air-admission opening in the plenum chamber, and means adjustably to fiex the diaphragm to control the convergence of the air-flow passage between said diaphragm and said plate structure from said admission opening to said perforations of the plate structure.

14. An air-distribution system as in claim 13 in which at least some of the air-flow devices have structure adjustable to vary the effective area of the central opening of the diaphragm to control the quantity of air discharged independently of the adjustment of the diaphragm for control of the velocity gradient of the discharged air.

References Cited in the file of this patent UNITED STATES PATENTS OLeary Feb. 4, 1890 Hart June 1, 1937 McKnight Mar. 7, 1939 Lead-better Aug. 5, 1941 Ericson Oct. 26, 1954 Ericson Oct. 1, 1957 Kennedy Feb. 4, 1958 Wakefield May 6, 1958 FOREIGN PATENTS Switzerland Jan. 15, 1958 

1. AN AIR-DISTRUBTION SYSTEM FOR SUPPLYING DIFFERENT PREDETERMINED QUANTITIES OF AIR TO DIFFERENT ZONES OF A BUILDING SPACE AND WITH PREDETERMINED DIFFERENT DEPTHS OF PENETRATIONS INTO DIFFERENT ZONES COMPRISING A WALL COMMON TO SAID DIFFERENT ZONES, A PLENUM CHAMBER SUBSTANTIALLY COEXTENSIVE WITH SAID WALL AND CONTINUOUSLY SUPPLIED WITH AIR FOR MAINTENANCE OF POSITIVE PLENUM PRESSURE, AND A PLURALITY OF AIR-CONTROL DEVICES INSERTED IN SAID WALL IN A PATTERN DEPENDENT UPON THE DIFFERENT AIRLOAD REQUIREMENTS OF DIFFERENT ZONES AND JOINTLY HAVING A DISCHARGE AREA NOT EXCEEDING ABOUT 2% OF THE AREA OF SAID WALL FOR SUBSTANTIAL UNIFORMITY OF THE PLENUM PRESSURE AT ALL OF SAID AIR-CONTROL DEVICES, SAID CONTROL DEVICES EACH COMPRISING A PERFORATED PLATE STRUCTURE BRIDGING AN OPENING IN THE WALL AND STRUCTURE, SUCH AS A PLATE, IN THE PLENUM CHAMBER AND HAVING A SURFACE VARIABLY SPACED ALONG SAID PERFORATED LOWER PLATE TO FORM A FLOW PASSAGE CONVERGING IN THE DIRECTION OF FLOW OF AIR FORM THE PLENUM THROUGH THE PERFORATIONS IN SAID PLATE AND DETERMINING THE FLOW RESISTANCE OF THE AIR-FLOW PATHS TO SAID PERFORATIONS, AT LEAST SOME OF SAID CONTROL DEVICES HAVING MEANS FOR ADJUSTING THE INTERPLATE SPACING. 